This invention relates to the optimized rolling of steel, particularly microalloyed steel.
In an as-hot rolled microalloyed steel, optimum strength and toughness are conferred by a fine grained polygonal ferrite structure. Additional strengthening is available via precipitation hardening and ferrite work hardening, although these are generally detrimental to the fracture properties. The development of a suitable fine grained structure by thermomechanical processing or working such as hot rolling, can be considered to occur in three or rarely four stages or regions. In the first, a fine grained structure is produced by repeated austenite recrystallization at high temperatures. This is followed, in the second, by austenite pancaking at intermediate temperatures. The third stage involves the still lower temperatures of the intercritical region, i.e. the ferrite/austenite two-phase range. Rarely, further working below the ferrite/austenite two-phase temperature range can occur. The final microstructure is dictated by the amounts of strain applied in each of these stages.
The first stage occurs at temperatures above a critical temperature T.sub.n, being the temperature below which there is little or no austenite recrystallization. The second stage occurs at temperatures below temperature T.sub.n but above another critical temperature A.sub.r3, being the upper temperature limit below which austenite is transformed into ferrite. The third stage occurs at temperatures below temperature A.sub.r3 but above another critical temperature A.sub.r1, being the lower temperature limit below which the austenite-to-polygonal ferrite transformation is complete. The final stage occurs below temperature A.sub.r1 (The designations A.sub.r3 and A.sub.r1 are generally used to identify the upper and lower temperature limit respectively of the ferrite/austenite two-phase region, as it exists during cooling.) Since no useful improvement in steel characteristics normally occurs below temperature A.sub.r1, steel is not ordinarily rolled below this temperature, although further such rolling would tend to further harden the steel.
Some basic principles of rolling schedule design are known. It is known, for example, that beneficial results are obtained by straining the steel to a significant extent in the intercritical region between temperatures A.sub.r3 and A.sub.r1 : Matrosov et al., "Influence of Incremental Deformation in Gamma Plus Alpha and Alpha Regions on Mechanical Properties of 0962 Steel" (1979) 11 Izvestiya VUZ Chernaya Metallurgiya 115. Tanaka et al. have recognized that the three useful stages of deformation occurring respectively above temperature T.sub.n, between temperatures T.sub.n and A.sub.r3, and between temperatures A.sub.r3 and A.sub.r1 can be analyzed to facilitate design of a useful rolling schedule: Tanaka et al., "Three Stages of the Controlled Rolling Process" Microallying '75, Union Carbide, Washington, D.C., 1975, p. 107.
In order to design a rolling schedule to produce desired mechanical properties in the steel, the temperature ranges or regions over which the three normally useful stages of deformation occur must be reasonably accurately known. However, the critical temperatures T.sub.n, A.sub.r3 and A.sub.r1 are not known a priori from the steel composition--rather, they are themselves also dependent on the rolling schedule. The rolling schedule details must therefore be known to some extent before the temperature limits of the three regions can be defined.
Heretofore, steel rolling schedules have been determined on an empirical basis typically involving a good deal of trial and error. It has not been possible to derive predictable quantitative relationships between desired steel properties and rolling mill operating parameters. In many cases the result has been that an appreciable proportion of steel production has not met specifications, especially where specifications are high and a fairly narrow "window" of acceptable mill operating conditions sufficient to enable specifications to be met exists.